VẬT lý địa CHẤN 12 freq

Overview
Fundamentals
Introduction
Seismic waves: Propagation Velocity and Amplitudes
Seismogram
Measurement systems
Sources, receivers, Acquisition strategies
Data processing
“Pre-processing”
Filter und Deconvolution
Velocity analysis and Stacking
Migration
Interpretation


Basic scheme of the seismic data processing
Loading of the data/conversion
Demultiplexing
Geometry

Measurements
Navigation

Pre-Processing

Editing
Amplitude correction
Frequency filter
Deconvolution
CMP-Sorting

Prestack

Velocity analysis
NMO-Correction
Stacking
Migration

Poststack

Post-processing

Printing/storage

Printing/storage


Frequency analysis
• Analysis of the frequency content of the
data
• Difference in frequency content for
interface waves, Noise and Reflections
• In what frequency range are the reflections
present ?


Outline
• Periodic and transient waveforms
• Fourier transformation
time domain <-> frequency domain
• Frequency filters

• Tapering
• Field examples


Frequency content of a shot
high freq.
noise:

low freq.
ground-roll


Waves
Amplitude
Phase φ

Acos(2πft)
Acos(2πft-φ)

t

1
Period τ ==--f

Period τ
Wavelength λ=τ∗v

2π
Wavenumber k = --λ

Velocity v

=

f⋅λ

2π
-----ω


Periodic and transient waveforms

Periodic waveform

Transient waveform


Periodic waveforms
• Any periodic waveform, may be
decomposed into a series of sine (or cosine)
waves whose frequencies are integer
multiples of the basic repetition frequency
• It is necessary to define not only the
frequency of each component, but also its
amplitude and phase


Fourier transformation
Fourier Transformation:

∞

G( f ) =

−i 2 πft
(
)
g
t
e
dt
∫

−∞

G( f ) =

∞

∞

−∞

−∞

∫ g (t )cos(2πft )dt − i ∫ g (t )sin(2πft )dt
∞

Inverse Fourier
Transformation:

g (t ) = ∫ G ( f )e i 2πft df
−∞

∞

∞

−∞

−∞

g (t ) = ∫ G ( f ) cos(2πft )df + i ∫ G ( f )sin( 2πft )df


Summation of different frequencies


Fourier transformation

g(t)

A(f)

Amplitude-Spectrum

φ(f)

Phase-Spectrum

G(f) = A(f) e iφ(f)

where A(f)={[Real part of G(f)]2+[Imaginary part of G(f)]2}1/2
Φ (f)=tan-1

(

imaginary part of G(f)
real part of G(f)]

)
A(f)

Imaginary part

Φ (f)

Real part


• frequency
• amplitude
• phase
f1
f2

Sum:

f1

f2

Sum:


f1

f1

f2

f2

Sum:

Sum:


Digital representation of continuous amplitude and phase spectrum
associated with a transient waveform


Spike function

DC bias

Transient waveforms
approximating
seismic pulses

Frequency filter
Low Pass

High Pass

f

f
Notch filter

Band Pass

f

f


Tapering


Tapering of filters
• Butterworth
• Hanning
• Ormsby
Amplitude and phase characteristics.


Band-pass filtered raw field records

Corrected for geometric spreading: frequency absorption remains